Download Making Sense Of Oxygen Sensors

Counter Point
Volume 2 Issue 3, September 1998
T H E
E L E C T R O N I C,
D I A G N O S T I C
A N D
D R I V E A B I L I T Y
Making Sense Of
Oxygen Sensors
T
R E S O U R C E.
Counter Point
Commemorates
Its First Year!
With this issue, WELLS Counter Point newsletter
oday’s computerized engine control systems rely
marks the conclusion of its well-received first year
of publication, a year in which more than 20,000
subscription requests were received.
on input from a
“The favorable response from technicians and
others involved in automotive repair has been even
variety of sensors
more than we expected,” said Gavin Spence, Vice
President of Sales for WELLS Manufacturing Corp.
to regulate engine performance,
“Our goal from the start has been to make
Counter Point a valuable automotive diagnostic
emissions and other important functions.
resource, presented in a straightforward, easy-toread style. We’re pleased that so many technicians
have told us we achieved that. We’ll work to make
These sensors must provide accurate information
otherwise driveability problems, increased fuel
consumption and emission failures can result.
One of the key sensors in this system is the oxygen
(O2) sensor. The O2 sensor is mounted in the
exhaust manifold to monitor unburned oxygen in
the exhaust. The O2 sensor tells the computer if the
fuel mixture is burning rich (less oxygen) or lean
(more oxygen). The computer then uses the sensor’s
voltage signal to alter the fuel mixture, creating a
feedback loop that constantly rebalances the fuel
mixture.
When the engine is first started, the computer is in
“open loop” and does not use the signal from the O2
sensor. The fuel mixture is set to run rich and stays
that way until the system goes into “closed loop” and
starts using the O2 sensor signal to
vary the fuel mixture. Most late-model O2
sensors are “heated” so they will warm up and reach
operating temperature sooner to reduce emissions.
Heated O2 sensors typically have
three or four wires. Older single-wire O2 sensors
do not have heaters.
An O2 sensor failure or a problem in the sensor’s
wiring circuit will prevent the system from going into
closed loop causing a rich-fuel condition, increased
emissions (particularly carbon monoxide) and fuel
consumption. Keep in mind that a bad coolant
temperature sensor can also prevent the system from
going into closed loop.
Counter Point even more useful in the future.”
HOW THE O2 SENSOR WORKS
One quick note: we’ve changed the publication
The O2 sensor used in most vehicles is a voltage
generating sensor. The tip of the sensor has a
zirconium ceramic bulb coated on the inside and
outside with porous platinum, which serve as the
electrodes. Inside the bulb are two platinum
electrodes. The inside of the bulb is vented through
the sensor housing to the outside atmosphere.
date for this issue from August to September, to
When the bulb is exposed to hot exhaust, the
difference in oxygen levels across the bulb creates a
voltage. The sensor will generate up to about 0.9 volts
when the fuel mixture is rich. When the mixture is
lean, the sensor’s output voltage can
drop as low as 0.1 volts. When the air/fuel mixture is
balanced (about 14.7:1), the sensor will generate
around 0.45 volts.
We at WELLS look forward to being a valuable
Despite the tremendous response already,
we will continue to honor subscription requests.
Please mail them to WELLS, P.O. Box 70, Fond du Lac,
WI 54936-0070. We also welcome your comments
and technical questions.
put us on a calendar-year quarterly schedule.
Because of many requests, we’ll also three-hole
drill Counter Point so you can save the issues
in a binder.
Once again, thank you for your support.
diagnostic and driveability resource to you for
many years to come.
continued on page 2
WHAT’S INSIDE: Making Sense of Oxygen Sensors…Page 1 / Counter Point Celebrates Its First Year…Page 1 / FINE TUNING…Page 2 /
QUALITY POINTS: WELLS Premium Spark Plug Wire Sets…Page 3 / HOT OFF THE WIRE: Isolation Diagnostic Charts…Page 4 / Publisher’s Information…Page 4
continued from page 1
Making Sense Of Oxygen Sensors
When the engine is running in closed loop, a good
O2 sensor will produce a voltage signal that is
constantly changing back and forth from rich to lean.
The transition rate is slowest on engines with
feedback carburetors, typically one per second at
2500 rpm. Engines with throttle-body injection
are somewhat faster (two or three times per second at
2500rpm), while engines with multiport injection are
the fastest (five to seven times per second at 2500 rpm).
OBD II = MORE O2 SENSORS
On late model vehicles equipped with OBD II, the
number of oxygen sensors used has doubled. A
second oxygen sensor mounted behind the catalytic
converter is used to monitor converter operating
efficiency. The OBD II system does this by
comparing the fore and aft O2 sensor readings. Little
or no change in oxygen readings between both sides
of the converter can mean the converter isn’t doing
anything. This may cause the Malfunction Indicator
Lamp (MIL) to come on.
O2 SENSOR DIAGNOSIS
O2 sensor performance diminishes with age as
contaminants accumulate on the sensor tip and
gradually reduce its ability to produce voltage. The
sensor becomes sluggish and takes longer to react to
oxygen changes in the exhaust causing emissions and
fuel consumption to go up. The effect is most
noticeable on engines with multiport fuel injection
(MFI) because the fuel ratio changes more rapidly
than on the throttle-body or feedback carburetor
applications. If the O2 sensor fails, the computer may
go into open loop operation. Contaminants include
lead, silicone (from internal coolant leaks or the
wrong type of RTV sealant), phosphorus (from oil
burning), and sulfur.
The operation of the O2 sensor should be checked
any time there’s an emissions or engine performance
problem. The sensor should also be checked when the
spark plugs are replaced. Some vehicles (imports
mostly) have a maintenance reminder light for
checking the O2 sensor.
Reading the O2 sensor’s output voltage while the
engine is running (at normal temperature) with a scan
tool or digital voltmeter will tell you if the sensor is
producing a signal, but not if the signal is a good one
because the numbers jump around so much. A highimpedance analog voltmeter is better for viewing
transitions, but may not respond quickly enough on
systems with higher transition rates. So the best
instrument for observing the O2 sensor’s voltage
output is a digital storage oscilloscope (DSO). A
scope will display the sensor’s voltage output as a
wavy line that shows both its amplitude (minimum
and maximum voltage) as well as its frequency
(transition rate from rich to lean).
Fine Tuning questions are answered by
Jim Bates, Technical Services Director.
Please send your questions to: Jim Bates
⁄ WELLS Manufacturing Corp.,
P.O. Box 70, Fond du Lac,
WI 54936-0070 or e-mail him at
[email protected]. We’ll send
you a WELLS’ shirt if your question is
published. So please include your shirt
size with your question.
co
Fine Tuning
Q: “We have a 1988 Pontiac LeMans that is
hard to start during rainy, damp
weather. Replaced the spark plugs and
plug wires which help somewhat, but
problem still occurs. What’s up?”
connection. But if the female connectors on the ends of
the pick-up coil wires are loose or corroded, the problem
will soon reappear.
Charley Meyers, Ft. Wayne, IN
Hard starting can be caused by condensation on the
inner and outer surfaces of the distributor cap.
Moisture drains off the ignition voltage and weakens
the spark making the engine difficult or impossible to
start. The buildup of moisture on the cap on this car
can be made worse by the plastic-bag style splash shield
that covers the cap on vehicles built before VIN
JB372711. Discard the splash shield, then remove the
cap, wipe it dry and carefully inspect it for wear,
burning, cracks or carbon tracks. Replace the cap
and rotor if any defects are found.
Q: “A 1987 Ford Thunderbird with a 3.8L
engine idles rough and sometimes
stalls. Checked intake vacuum, spark
plugs, ignition timing, fuel pressure and
compression. All are okay.”
Disconnect the green and white pick-up coil wires from
the HEI module in the distributor. Check the continuity
of the pick-up coil windings by connecting an
ohmmeter across the two terminals. A good winding
with tightly crimped terminals will produce a reading
between 500 and 1500 ohms.
Recrimping the connectors on the wires and/or soldering
the connectors to the wires will help assure good contact.
Also, make sure the pick-up coil-wire female connectors
fit the spades on the HEI module tightly. Apply
dielectric grease to the terminal connections to prevent
oxidation.
Jose Raffiel, San Diego, CA
An erratic idle, rough running and stalling on this car
may be caused by chafing of the MAP sensor wires.
The wires may rub against the engine-ground strap,
creating a short, which may also set a code 22. Loosen
the engine ground-strap attaching screw and rotate the
eyelet to a downward position so the strap does not
make contact with the MAP sensor wires. Tighten the
ground-strap screw and repair the insulation on the
MAP sensor wiring.
Q: “A 1991 Chevy Caprice with HEI ignition
dies and won’t restart (no spark).
Replaced ignition module. Engine ran
fine for several days, then died (again,
no spark).”
Q: “A 1995 Honda Civic misfires badly when
accelerating. One spark plug was found
to be wet. Replaced spark plugs.
Condition improved but still misfires
occasionally under load.”
Wade Jackson, Lubbock, TX
Check the spark plug wires. Excessive resistance in the
plug wires can increase the firing voltage requirements to
the point where the spark may not be able to jump the
electrode gap when the engine is under load.
Jeff Steiner, Columbus, OH
Don’t be too quick to blame the module. The problem
may be a faulty pick-up coil connector. The pick-up coil
produces a small current that the module uses to switch
the ignition coil on and off. If a poor connection exists
in the pick-up coil circuit, the module may lose the
trigger signal causing the engine to die because of no
spark. Repositioning the pick-up terminal leads when
the module is replaced may temporarily restore the
continued from page 3
2
Loose-fitting spark plug boots or damaged insulation on
the plug wires may also provide paths where the spark
can escape to ground. Good plug wires should generally
have less than 8,000 ohms per foot resistance. If
resistance exceeds specifications, or the plug wires are
found to be damaged, replace the wires.
A SIMPLE WAY TO TEST AN O2 SENSOR
continued from page 2
Making Sense Of Oxygen Sensors
The O2 sensor can also be checked by removing it
from the vehicle, connecting a digital voltmeter to
the sensor and using a propane torch to heat the
sensor element.
A good O2 sensor should produce an oscillating
waveform at idle that makes voltage transitions
from minimum (0.3v or less) to maximum
(0.8v or higher).
With a zirconia O2 sensor, connect the positive test
lead to the O2 sensors lead wire, and the negative
test lead to the sensor housing or shell (ground).
Hold the sensor with pliers, then use the propane
torch to heat the sensing element. The flame
consumes most of the oxygen, so the sensor should
generate a “rich” voltage signal of about 0.9 volts
within60 seconds or less.
Next, remove the flame while observing the
voltmeter. The sensor reading should drop to
less than 0.1 volt within three seconds if the sensor
is good.
Making the fuel mixture artificially rich by
feeding propane into the intake manifold should
cause the sensor to respond almost immediately
(within 100 milliseconds) and go to maximum
(above 0.8v) output. Creating a lean mixture by
opening a vacuum line should cause the sensor’s
output to drop to its minimum (0.3v or less)
value. If the O2 sensor voltage does not change
back and forth quickly enough, it needs to
be replaced.
To test the sensor’s heater circuit, connect an
ohmeter (set on low scale) across the sensor’s two
heater wires. If the heater circuit is okay, the meter
will register a resistance reading. The actual reading
is not important so long as the meter registers
something other than an open circuit.
HOW TO TEST A TITANIA O2 SENSOR
Next, remove the flame and note the reading again.
A good sensor should go back to infinity.
Some vehicles (Jeep & Toyota) have “titania” O2
sensors that change resistance rather than produce a
voltage. This type of O2 sensor can be tested with
an ohmmeter and propane torch.
A sluggish O2 sensor often won’t set a fault code,
so don’t assume the O2 sensor is okay if there is
no code.
Quality
Set the ohmmeter on the 200K scale, and connect
it across the black and gray sensor leads. Use the
torch to heat the tip of the sensor as before, and
note the ohmmeter reading. A good sensor should
show some resistance (the reading will vary with
flame temperature).
Points
Plug wires should always be inspected anytime an
ignition or driveability problem is experienced, and
every time the spark plugs are replaced.
WELLS Premium Spark Plug Wire Sets:
The Perfect Replacement For OEM
Unlike many aftermarket spark plug wire sets,
WELLS Premium Wire Sets are a perfect
replacement for OEM wire sets. WELLS wire sets
duplicate the appearance, fit and performance of all
OEM wire sets in every way. Wire lengths are the
same as the original, and some sets even include
two different lengths of coil wire so that there are
no compromises.
The same high-quality, high-temperature materials
are used in WELLS Premium Wire Sets for the
conductor, insulation, jacketing and boots as the
OEM wires -- and the boots are designed to not
only duplicate the look and fit of the OEM boots
but also their performance.
For example, the WELLS QW942 replacement wire
set for the 1994-95 Ford Explorer with 4.0L V6 and
DIS ignition have the same pinkish-orange high
temperature silicone boots as the original wire set.
Many aftermarket wire sets do not have special high
temperature boots for applications that require
them. Yet many late-model Ford and Chrysler
applications have wire sets that require them.
On Ford, black silicone boots provide
protection up to 450 degrees, gray
boots are for applications that require
up to 550 degrees, and the pinkish-orange
boots are for ones that must withstand
up to 600 degrees.
WELLS Premium Wire Sets are backed
by a lifetime limited warranty.
When should plug wires be replaced? New wires
are needed if the wires have been damaged by
mishandling or the wires are shorting, arcing,
chaffing or burning. Wires should also be replaced
if the resistance per foot exceeds the OEM
specifications for the application, generally no more
than 8,000 ohms per foot.
3
When replacing wires, start with the longest wire first.
It’s important to make sure the spark plug terminal is
snap-locked in place (listen for an audible “click”) to
prevent arcing which could lead to premature failure
of the wire as well as the ignition coil. Wires must also
be routed the same as before to prevent crossfire or
electromagnetic interference, and properly supported
to prevent vibration, chaffing or contact with hot
exhaust manifolds.
WELLS MANUFACTURING CORP.
P.O. Box 70
Fond du Lac, WI 54936-0070
INSIDE:
ENSE OF
MAKING S
ENSORS
OXYGEN S
Publisher’s Information
Hot off the Wire
WELLS’ President ............William Allen
Vice President Sales ........Gavin Spence
Technical Services Director .....Jim Bates
WELLS’ Website Has Isolation Diagnostic Charts
If you’re looking for tips on how to test various
kinds of sensors and other ignition parts, check out
the technical tips on WELLS’ website at
www.wellsmfgcorp.com.
Here you will find
diagrams showing how to
isolate faults in oxygen
sensors, ignition coils,
hall effect sensors, temperature sensors, throttle
position sensors, mass air flow sensors, frequency
generating MAP sensors and knock sensors.
Each chart shows
how to hook up test
equipment and check
the component.
Newsletter Editor ...............Ron Raposa
Counter Point is a quarterly publication of WELLS
Manufacturing Corp., P.O. Box 70, Fond du Lac, WI
54936-0070. Letters and comments should be
directed to: Ron Raposa, Counter Point Editor, 3602
Inland Empire Blvd, Suite C200, Ontario, CA 91764.
© COPYRIGHT 1998 WELLS MANUFACTURING CORP.
All rights reserved. No reproduction in whole or part is
permitted without the written consent of WELLS Manufacturing
Corp. Requests should be faxed to Ron Raposa at (909) 941-0877.